I'm an accountant and so naturally I keep a record of all my spending. Tonight out of curiosity I looked into how much I've spent on various games over the last few years and I found I've spent more on MH than I had previously thought. It was also clear why the game is "free" and not subscription-based. Since MH has launched I've spent about 3 times as much on the game as I would have than if MH used an old school model of $60 box & $15/month.
I don't regret my purchases and don't plan to change my spending habits on MH, I just thought it was enlightening.
Also, people who think $15/month is too much for an online game should be careful what they wish for. "Free" doesn't mean cheaper. :-)